1. Field of the Invention
The present invention relates to a power supply apparatus and an image forming apparatus, and more particularly, to realize soft start and solve a problem of noise in a ringing choke converter.
2. Description of the Related Art
A ringing choke converter (hereinafter referred to as “RCC”), which is a self-excited circuit, is formed only of discrete components and is inexpensive, and is therefore widely used as a power supply of 150 W or less. FIG. 8 illustrates a circuit example of a conventional RCC. The RCC of FIG. 8 includes a primary smoothing capacitor C51, capacitors C52 to C54, transistors Q52 and Q53, a MOSFET Q51 (hereinafter referred to as “main switch element Q51”), and a diode D51. The RCC of FIG. 8 further includes a flyback transformer T51, a photocoupler PC51, an operational amplifier OP51, a Zener diode VZ51, and resistors R501 to R509. As a power saving technology for the RCC, there is known a method of applying a pulse signal to the transistor Q53 to forcibly stop self-excited continuous oscillation and change to intermittent oscillation so as to increase efficiency in a light load state, as disclosed in Japanese Patent No. 3697218, for example.
The conventional RCC has a feature that a drain-source voltage of the main switch element Q51 becomes highest when the power supply starts up. The RCC determines a maximum ON-time of the main switch element Q51 based on a time constant of the resistor R504 and the capacitor C53. Once the main switch element Q51 is turned ON, the capacitor C53 is charged with a voltage generated in an auxiliary winding of the flyback transformer T51. Then, when the voltage of the capacitor C53 exceeds a threshold of a base-emitter voltage of the transistor Q52, the transistor Q52 is turned ON and the main switch element Q51 is turned OFF.
In general, in the state in which an output voltage is obtained from the secondary side of the RCC, feedback control is performed by the operational amplifier OP51 so that a charge current flows to the capacitor C53 also via the resistor R509, and hence the ON-time of the main switch element Q51 is controlled to be shorter. Even when the output voltage reaches a maximum continuous output of the RCC, the capacitor C53 is charged because of minute light emission of an LED of the photocoupler PC51 obtained by an output offset voltage of the operational amplifier OP51, and hence the ON-time of the main switch element Q51 does not reach the maximum ON-time determined only by the resistor R504 and the capacitor C53. At the start-up of the power supply, however, the secondary side output voltage has not risen yet, and hence the ON-time of the main switch element Q51 becomes the maximum ON-time determined only by the resistor R504 and the capacitor C53. Thus, a peak value of a drain current flowing when the main switch element Q51 is turned OFF becomes maximum, and hence a surge of the drain-source voltage of the main switch element Q51 caused by leakage inductance of a primary winding of the flyback transformer T51 becomes maximum as well.
To deal with this, the values of the resistor R504 and the capacitor C53 are set so that the drain-source voltage on this occasion may satisfy the absolute maximum rating of the main switch element Q51. As a result, the maximum output after the start-up becomes an output that can be supplied in an ON-time shorter than the maximum ON-time at the time of start-up. In other words, the performance cannot be fully utilized. If the maximum ON-time at the time of start-up can be set to be equal to or shorter than a maximum ON-time under a maximum load after the start-up, the maximum output can be increased even with the same MOSFET and the same transformer, which is advantageous.
Another problem is that humming noise is heard from the transformer when the intermittent oscillation operation is performed as disclosed in, for example, Japanese Patent No. 3697218. The intermittent oscillation operation means the repetition of start-up and stop, and the main switch element Q51 is driven with the above-mentioned maximum ON-time at the time of start-up, and hence a change in magnetic flux of the main transformer becomes the largest. The oscillation frequency during the start-up decreases to enter the audible range and becomes audible by a user as raspy noise. In general, countermeasures for noise are taken by impregnating the transformer with epoxy or the like. However, this effect is limited and the cost is increased, and hence the advantage of the RCC as being inexpensive is lost.